The story begins with Romans. The first structure on this site was a bath — thermae — of the kind that Roman cities built as civic infrastructure and social institution. After the baths fell out of use, a hall church rose in their place, probably in the sixth century; coin finds in stratigraphic excavation pin the date with unusual precision. Sometime in the seventh century, a larger church was built to the south of the first. Then, at the end of the twelfth century, a third church replaced or incorporated its predecessor. This final Byzantine church may date to between 1197 and 1204: the scholar Constantine Stilbes mentioned its destruction in a fire in 1197, which suggests either that the dating is off or that reconstruction was swift.

After the Fourth Crusade seized Constantinople in 1204, the church passed to the Latin conquerors. Franciscan clergy celebrated Mass here for several decades — a detail that makes the building's later history all the more striking. In 1453, when Mehmed II took Constantinople, he assigned this church personally to the Kalenderi sect of the Dervish. The Dervishes used it as a *zaviye* (a Sufi lodge) and an *imaret* (a public kitchen). From the name of that order the building took the name it still carries: Kalenderhane, meaning, roughly, the house of the Kalenders. A mihrab, minbar, and mahfil were added to complete its conversion to a mosque.

The building survives in a form recognizable to its Byzantine builders, though the Ottoman additions have become part of its character. The plan is a central Greek cross with deep barrel vaults over each arm and a dome above the crossing — the dome has sixteen ribs, a technical refinement typical of middle Byzantine construction. The masonry alternates layers of brick and stone in the style that Byzantine builders used across the eastern Mediterranean. Entry today is through an esonarthex and an exonarthex on the west side. An upper gallery once occupied the space above the esonarthex, following the same arrangement found in the great Church of the Pantokrator; it was removed in 1854. Alongside the Gül Mosque in Istanbul, the Church of Hagia Sophia in Thessaloniki, and the Church of the Dormition in Iznik, Kalenderhane stands as one of the principal surviving examples of the domed Greek cross plan from the Byzantine middle period.

The excavations that began in the 1960s did more than reveal the building's structural history. Restorers discovered two frescoes bearing the Greek word *Kyriotissa* — meaning enthroned — in the southeastern chapel and above the main entrance to the narthex. The find resolved a long-standing mystery about the church's original dedication: it was almost certainly dedicated to the Theotokos Kyriotissa, the Enthroned Mother of God. A separate fresco cycle depicting scenes from the life of St. Francis of Assisi was also uncovered — among the oldest known representations of the saint, painted only a few years after his death in 1226. Both fresco groups have been detached, partially restored, and transferred to the Istanbul Archaeology Museums, where they can be seen today. What the walls of Kalenderhane once showed, the museum now preserves.